Conventional Hall-Heroult cells for the electrolytic production of aluminum employ a carbon cell bottom which serves to supply current to a deep pool of molten aluminum forming the cathode. The cathodic aluminum is necessarily thick (at least 8-10 cm) because carbon is non-wettable by molten aluminum and would not completely cover the carbon if the aluminum layer were thinner. In the conventional arrangement, a horizontal steel conductor bar is embedded in the lower part of the carbon cell bottom for the supply of current from an external source. Thus, the entire cell bottom in contact with the molten aluminum cathode consists of carbon which, in operation, is impregnated with sodium species and other ingredients of the cryolite leading to the formation of toxic compounds including cyanides. Despite the many disadvantages associated with carbon as cathode current feeder material (non-wettability by aluminum, necessitating deep pool operation; the relatively high electrical resistance of carbon, leading to energy losses; reactions within the cell environment necessitating disposal of large quantities of contaminated carbon when the cell bottom is renewed, etc..), attempts to replace it with theoretically more advantageous materials employing new cell designs have not so far met with success.
Thus, for example, an aluminum production cell having an electrically non-conductive refractory lining with a "bottom entry" current collector is described in U.S. Pat. No. 3,287,247. The inner end of the current collector has a rounded cap of TiB.sub.2 projecting into a depression containing a deep pool of molten aluminum. U.S. Pat. Nos. 3,321,392 and 3,274,093 describe a similar arrangement in which the protruding ends of TiB.sub.2 conductor bars are rounded.
U.S. Pat. No. 3,156,639 describes a similar arrangement in which the TiB.sub.2 or other RHM cap is connected to a stem by a metal joint. In a variation a graphite block, of the general shape and dimensions of conventional pre-baked cathode blocks, has a curved upper surface covered by hot-pressed and bonded refractory boride material which contacts the molten aluminum. This diboride cap is surrounded by a refractory sleeve. In its lower part, i.e. adjacent the conventional horizontal conductor bar, there is a groove for a steel connecting rod. However, the necessary bonding of the refractory boride layer on the graphite body is very difficult to achieve and the arrangement is therefore impractical.
U.S. Pat. No. 4,613,418 has proposed an aluminum production cell with an alumina potlining in which bottom-entry current collectors are embedded and extend to a recess in the potlining. To prevent the unwanted collection of sludge in these depressions, this patent proposes filling the depressions with balls of aluminum-wettable material. Related designs are proposed in U.S. Pat. No. 4,612,103.
These alternative cell designs, using a non-carbon cell bottom, have great promise. Replacement of the carbon cell bottom with, e.g., alumina leads to potential savings in materials and operating costs. However, such proposals heretofore have all relied on the use of a family of materials known as Refractory Hard Metals ("RHM") encompassing the borides and carbides Of metals of Groups IVB (Ti, Zr, Hf) and VB (V, Nb, Ta) of the periodic table of the elements. TiB.sub.2 has been identified as the most promising RHM material. However, the use of these materials has encountered a number of problems including cost and the difficulty of producing and machining large pieces of the materials. Such difficulties have led to the design expedients proposed in the aforementioned U.S. Pat. Nos. 4,613,418 and 4,612,103, where, for example, small pieces of TiB.sub.2 are assembled or packed together in an environment of molten aluminum as part of the current supply arrangement.
The problems experienced with RHM current collectors and further expedients for dealing with them, namely the provision of a protective barrier incorporating a molten fluoride- or chloride-containing salt mixture or a getter such as particulate aluminum, are further described in EP-A-0 215 555.
A side entry design has been described in UK-A-1 127 313 in which graphite cathode blocks are connected to an external current supply via oxygen-free copper current collectors extending horizontally through the sides of a rammed carbon potlining, the graphite blocks extending into the cathodic pool of molten aluminum. Side entry designs however involve various drawbacks and have not found commercial acceptance.